Flat panel electronic displays have become more and more popular in many electronic media because of their flexibility, lightweight, and mechanically robust property. Such potential for flat panel electronic displays and other devices on plastic substrates has motivated considerable research on new materials and improved processes for fabricating TFTs.
The requirements and applications to the active devices for TFTs are getting more. Low-temperature film formation, less-selectivity on substrate, and direct patterning are the basic requirements in the fabricating process for the applications of TFTs. In addition, it requires the active devices to be suitable for the applications of both glass substrate and plastic substrate, in order to achieve the advantages of low fabrication cost and getting a large display area.
A recent attempt to fabcriate a TFT by a solution method has the advantages of easy and accurate control on material components, low-temperature fabricating process, high quality on the film, and low fabrication cost.
In general, TFTs can be catagorized into two types of structure, inverted staggered and co-planar. As shown in FIG. 1, the co-planar TFT structure has a gate electrode 101 formed on the top of an active channel layer 102. On the contrary, the inverted staggered TFT structure has an active channel layer 202 formed on the top of a gate electrode 201, as shown in FIG. 2. These TFTs are usually formed from a thin film of amorphous silicon (a-Si) or polycrystalline silicon (poly-Si). The a-Si and poly-Si thin films are typically deposited by sputtering or chemical vapor deposition (CVD) and low-pressure chemical vapor deposition (LPCVD), respectively. The deposition temperature of either CVD or LPCVD is relatively high (around 350° C. for CVD and 500° C.-650° C. for LPCVD). This not only requires high-energy consumption but also limits the choices of panel substrates and panel sizes. Therefore, a low processing temperature for fabricating TFTs is highly desired.
In order to meet the requirements of low-temperature processing, deposition of TFTs using chemical solutions was proposed. In 1997, organic semiconductors were adopted by Dimitrakopoulos et al in U.S. Pat. No. 5,946,551 to fabricate active channels of TFTs. The drawback of the organic TFTs is that their performance (e.g., carrier mobility and ratio of on-current and off-current Ion/Ioff) is much inferior to their a-Si counterparts.
TFTs deposited by chemical solution method using CdS or CdSe as an active semiconductor was disclosed by Gan et al in U.S. Pat. No. 6,225,149 in 1999. The active layer of the TFTs was first deposited by a chemical bath deposition (CBD) method. Then, the active semiconductor layer was patterned using lithography and etching process. The method for fabricating a TFT requires an annealing step at 400° C. In addition, the toxic materials involved in the fabrication process are one of the major concerns of their applications.
In order to meet the low processing temperature requirements of flexible substrates (e.g., plastic substrates) and to maintain a performance equivalent to their a-Si counterpart, ZnO TFTs fabricated by rf magnetron sputtering was proposed by Carcia et al in 2003 (Appl. Phys. Lett., Vol. 82, No. 7, 1117). Although low-temperature solution deposition of ZnO has gradually received attention in the research community, the applications of the deposition method to TFT fabrication have not been applied yet. It is still in the research state on the characteristics of semiconductors.
Based on both low-temperature requirements and environmental concerns, future development trend is believed to focus on metal oxides and high dielectric constant materials. The materials used for thin-film devices fabrication will play an important role and guide the direction of technology development.